The technical scope of the invention is that of processes enabling the control of the trajectory of a spinning projectile and namely the control of the trajectory of a projectile fired from a cannon.
Projectiles fired from cannons have, in theory, a well known trajectory. However, non reproducible external factors (such as wind, temperature and atmospherical pressure) or internal factors (such as initial velocity and aerodynamic coefficients) can influence the trajectory.
This results in deviations in the impact on the ground of the projectile which, at the current maximum artillery range (around 35 km) can approach, as standard deviations, 500 m in range and 150 m in direction.
So as to improve firing accuracy, it is known to correct the projectile's trajectory in range by deploying one or several aerobrakes. Patent FR2786561 describes such a solution. Standard deviations in accuracy are, at 35 km, of around a hundred meters in range as well as in direction.
Another known solution is to ensure the autonomous and continual command of the trajectory by means of aerodynamic elevons. These are controlled by an autopilot onboard the projectile. The standard deviations in accuracy may thus be reduced to around ten meters or so in range and in direction. Moreover, by using the gliding effect it is possible to notably increase the maximal range (which may exceed 65 km with no additional propellant).
These autonomous in flight projectiles are provided with a satellite positioning system (more commonly known as GPS or global positioning system) which allows them to self locate with respect to their trajectory. Before being fired, the projectile is programmed with the coordinates of the target. It thereafter determines for itself its actual position in flight and prepares, using data supplied by an on-board inertial unit and appropriate algorithms, the control commands for the elevons.
The projectile also incorporates an inertial unit, comprising accelerometers and gyros, which supplies (for a reference linked to the projectile) the non-gravitationial instantaneous spin and acceleration vector components to which the projectile is subjected.
Three integrations made using the spin instantaneous vector components may thus supply the Euler angles of the projectile (pitch, yaw and roll) required to prepare the control commands of the trajectory.
Such a solution is classically used in projectiles such a cruise missiles that do not spin, or spin very little, (sp in rate of around 0.5 revs/s) and which are only subjected to reduced longitudinal accelerations (of around 10 times gravitational acceleration g). These projectiles incorporate very accurate but costly inertial units.
Such inertial units cannot be used in cannon-fired projectiles, since they are subjected to substantial accelerations borne by the projectile during firing. Additionally, the cost of the inertial units used in such projectiles must be as low as possible.
Inertial units that can be used in cannon-fired projectiles are of average accuracy and present drift leading to statistically high error in roll position reaching up to 90°. It is thus impossible to pilot the projectile, which is unable to reach its target. This phenomenon is further accentuated when the auto-spin velocity of the projectile is not nil.
One previously envisaged solution consists in estimating the initial roll position by applying reference piloting commands, during a preliminary phase, and deducting the roll position of the velocity components measured by the GPS locater.
However, this process has the drawback of not ensuring the guidance of the projectile during the application phase of the reference piloting commands. This results in a diminution of performances (reduction in maximal range and deterioration of accuracy for a given range).